1. Field of the Invention
The present invention relates to a PTC (xe2x80x9cpositive temperature coefficient of resistancexe2x80x9d) composite material favorably used in, for example, a current-limiting element which controls fault current.
2. Description of Related Art
The electrical resistance of PTC materials increases sharply with increasing temperature within a particular temperature range. Therefore, PTC materials are used, for example, as current-limiting elements which control fault current in a circuit.
The best known PTC material is a barium titanate type ceramic whose electrical properties change at the Curie point thereof. With this PTC material, however, power loss is large because of its high room temperature resistivity and, moreover, the production cost is high. Hence, the industry began searching for other substances that exhibit the PTC property. As a result, it was discovered that composite materials made of a polymer matrix and a conductive filler exhibited the same type of PTC property possessed by the barium titanate type ceramic.
For example, a mixture consisting of particular proportions of a crystalline polymer (e.g. a polyethylene) as an insulator and conductive particles (e.g. carbon particles) as conductive paths formed in the polymer matrix, exhibits very low room temperature (i.e., 30xc2x0 C.) electrical resistance, and acts as a conductor as a result of insulator-conductor transition. Specifically, since the polymer has a thermal expansion coefficient that is much greater than that of the conductive particles, the crystalline polymer expands dramatically when the composite material is heated and the crystalline polymer is melted. As a result, the conductive particles forming the conductive paths in the polymer are separated from each other at the melting point temperature of the polymer, the conductive paths are cut, and the electrical resistance of the composite material increases sharply. That is, the composite material exhibits PTC behavior.
When an organic substance such as the above polymer or the like is used as a matrix in a PTC composite material, however, there has been a problem in that when high temperatures caused by fault current continue for a long time, the composite material is unable to exhibit its intended action because the organic substance is generally low in heat resistance. Conventional polymer composite materials also have a problem in that they allow no reliable repeated operation, because the resistance of the material after a trip condition does not return to the initial resistance. It is thus difficult to rely upon these composite materials in sensitive circuit applications.
Studies have also been conducted on composite materials made of a silica type matrix, such as quartz, cristobalite or the like, and conductive particles. Like barium titanate type ceramics, however, these materials are high in room temperature resistivity and thus cause a large power loss. These materials also have a transition temperature (i.e., trip-point temperature) greater than 200xc2x0 C., thus making their use in certain circuit applications inappropriate.
In view of the above-mentioned problems with the prior art, the present invention has been completed to provide a reusable PTC composite material which has low room temperature resistivity, large resistivity jump at the transition temperature thereof, a transition temperature less than 200xc2x0 C., high heat resistance, and low power loss.
According to the present invention, there is provided a PTC composite material comprising (i) a matrix of ceramic material having one of a cristobalite crystal structure and a tridymite crystal structure, doped with an oxide of at least one of Be, B, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge and W, and (ii) a conductive phase dispersed throughout the matrix, the conductive phase comprising at least one of a metal, silicide, nitride, carbide and boride. The ceramic material preferably is one of cristobalite phase SiO2, tridymite phase SiO2, cristobalite phase AlPO4, and tridymite phase AlPO4.
Doping the matrix phase with the above-listed materials, preferably in an amount of 0.1 mol % to 20 mol %, results in a decrease in the transition temperature (i.e, the trip-point temperature) of the material to a level not greater than 200xc2x0 C., while maintaining a room temperature resistivity of less than 1 xcexa9cm. Moreover, the xe2x80x9cresistivity jumpxe2x80x9d at the trip-point temperature (i.e., the increase in resistivity at the trip-point temperature) of the material is at least 10, preferably at least 100, more preferably at least 1000, and most preferably at least 10,000 (and in some cases as high as 109).
The conductive phase of the PTC material typically takes the form of particles selected from the materials described above. Silicides of Ti, Zr, V, Nb, Ta, Cr, Mo, W, Co, and Fe are preferred conductive materials. These materials increase the cycle life of the PTC material, since silicides create a strong chemical bond with the matrix material and thus enhance the overall strength of the composite material. Silicides of Mo, Ta, W, Cr and Nb are most preferred, since those materials are also stable in air at high temperatures, and are thus less prone to deterioration during the high temperature operations used to form the composite material (e.g., the composite material can be dewaxed in an air atmosphere).
The average size of the conductive particles that form the conductive phase in the composite material is preferably 5 to 100 xcexcm, more preferably 20 to 60 xcexcm. If the average particle size is less than 5 xcexcm, the room temperature resistivity of the resultant material tends to be too high, and the resistivity jump at the trip-point temperature tends to be too low. If the average particle size exceeds 100 xcexcm, however, the cycle life of the composite material is degraded, as the stress due to thermal expansion mismatch between the matrix material and the conductive phase becomes too large. It is preferred that the conductive phase be present in an amount of 10 to 45% by volume (with respect to the total volume of the composite material), more preferably 20 to 35% by volume.
The PTC composite material also preferably has a relative density of 90% or higher, in order to lower the room temperature resistivity of the composite material, as porosity essentially acts as an insulating phase in the material. It is also preferred that, at the trip-point temperature of the composite material, the material expand from 0.2% to 1.4% (by volume). If the volume expansion is less than 0.2%, the composite material does not exhibit sufficient resistivity jump at the trip-point temperature. If the volume expansion is more than 1.4%, the composite material may experience stress cracking at the interface between the matrix and conductive phase.
It is also preferred that the composite material is produced at a firing temperature at least 20xc2x0 C. lower than the melting point of the lowest melting point material contained in the conductive phase of the composite material. This will maintain the position of the conductive particles making up the conductive phase during the firing operation. If the conductive particles were allowed to melt and agglomerate, there is a concern that relatively high conductivity areas may be formed through the composite material, and thus substantially decrease the high temperature resistivity of the composite material. Moreover, melted conductive particles could escape the matrix, thus making it difficult to control the volume ratio of conductive particles in the composite material.